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Quark

The proton that holds the nucleus of every atom you have ever touched is not a solid sphere but a chaotic, seething storm of three quarks bound together by gluons. These quarks, specifically two up quarks and one down quark, are the fundamental constituents of matter, yet they have never been observed in isolation due to a phenomenon known as color confinement. This invisible force ensures that quarks are forever locked inside composite particles called hadrons, such as protons and neutrons, meaning that all commonly observable matter in the universe is composed of up quarks, down quarks, and electrons. The existence of these particles was not immediately accepted by the scientific community when the theory was first proposed, as many physicists viewed them as mere mathematical abstractions rather than physical entities. It was not until the late 1960s that deep inelastic scattering experiments at the Stanford Linear Accelerator Center provided the first concrete evidence that protons contained smaller, point-like objects, fundamentally changing our understanding of the subatomic world.

The Birth Of A Theory

In 1964, two physicists working independently of one another, Murray Gell-Mann and George Zweig, proposed a radical new model to explain the chaotic 'particle zoo' of hadrons that had been discovered in the decades prior. Gell-Mann, a Nobel laureate who had previously formulated the Eightfold Way classification system, and Zweig, a young theoretical physicist, posited that these hadrons were not elementary but were instead composed of combinations of quarks and antiquarks. The initial reaction from the physics community was deeply skeptical, with many colleagues arguing that quarks were simply a convenient fiction used to organize data rather than real physical objects. Gell-Mann famously chose the name 'quark' from James Joyce's 1939 book Finnegans Wake, specifically from the line 'Three quarks for Muster Mark,' though he was initially undecided on the spelling and even considered the German word for curd cheese. Zweig, on the other hand, preferred the name 'ace' for the particle he had theorized, but Gell-Mann's terminology eventually gained prominence once the quark model was widely accepted. The discovery of the charm quark in 1974, known as the November Revolution, finally convinced the physics community of the model's validity, transforming quarks from a theoretical construct into a physical reality.

The Six Flavors Of Matter

The universe contains six distinct types, or flavors, of quarks, arranged into three generations that dictate their mass and stability. The first generation consists of the up and down quarks, which have the lowest masses and are the only ones that occur commonly in nature, forming the stable protons and neutrons that make up the atomic nuclei of the visible universe. The second generation includes the strange and charm quarks, while the third generation comprises the top and bottom quarks, which are significantly heavier and decay rapidly into the lighter first-generation particles. The top quark, discovered at Fermilab in 1995, is the most massive of all known elementary particles, with a mass almost as large as that of a gold atom, yet it decays so quickly that it does not have time to form hadrons. Heavier quarks can only be produced in high-energy collisions, such as those involving cosmic rays or in particle accelerators, and they were thought to have been present only during the first fractions of a second after the Big Bang. Despite extensive searches, no evidence has been found for a fourth generation of quarks, and the Standard Model posits that only three generations exist, with the fourth generation neutrino constrained to have a mass greater than approximately 45.5 GeV.

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Common questions

What are quarks and how do they form protons and neutrons?

Quarks are fundamental constituents of matter that combine to form protons and neutrons. A proton consists of two up quarks and one down quark bound together by gluons, while neutrons contain one up quark and two down quarks. These particles are never found in isolation due to color confinement.

Who proposed the quark model and when was it first experimentally confirmed?

Murray Gell-Mann and George Zweig independently proposed the quark model in 1964. Deep inelastic scattering experiments at the Stanford Linear Accelerator Center provided the first concrete evidence for quarks in the late 1960s. The discovery of the charm quark in 1974, known as the November Revolution, finally convinced the physics community of the model's validity.

How many types of quarks exist and what are their names?

The universe contains six distinct types or flavors of quarks arranged into three generations. The first generation includes up and down quarks, the second generation includes strange and charm quarks, and the third generation includes top and bottom quarks. No evidence has been found for a fourth generation of quarks.

What is color charge and how does it affect quark behavior?

Color charge is a property of quarks that serves as the source of the strong interaction binding them together. This property is labeled as red, green, and blue, and the combination of these charges must always result in a color-neutral state. The theory describing this interaction is called quantum chromodynamics and involves the exchange of gluons.

How was the mass of quarks discovered and what role do gluons play?

Most of a hadron's mass comes from the gluons that bind the constituent quarks together rather than from the quarks themselves. A proton has a mass of approximately 938 MeV, of which the rest mass of its three valence quarks only contributes about 9 MeV. The remainder is attributed to the field energy of the gluons known as quantum chromodynamics binding energy.

What is quark-gluon plasma and when did it exist in the universe?

Quark-gluon plasma is a theoretical phase of matter where quarks become deconfined and propagate as thermalized free excitations. This state is believed to have existed in the period prior to 10^-6 seconds after the Big Bang. Recent experiments at the Relativistic Heavy Ion Collider have yielded evidence for liquid-like quark matter exhibiting nearly perfect fluid motion.

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The Color Charge Mystery

Unlike any other elementary particle, quarks possess a property called color charge, which is the source of the strong interaction that binds them together. This property is not related to visible light but is arbitrarily labeled as red, green, and blue, with corresponding anticolors for antiquarks, and the combination of these charges must always result in a color-neutral or 'white' state. The theory describing this interaction is called quantum chromodynamics, and it involves the exchange of force-carrying particles known as gluons, which themselves carry color charge and can emit and absorb other gluons. This unique behavior leads to a phenomenon called asymptotic freedom, where the binding force between quarks weakens as they come closer together but strengthens as they move apart, much like an elastic band that becomes harder to stretch the further you pull it. Consequently, quarks are never found in isolation, and any attempt to separate them results in the creation of new quark-antiquark pairs, a process known as color confinement. The mathematical framework for this interaction is based on the gauge symmetry group SU(3)c, which dictates that the physics of quantum chromodynamics is independent of which directions in three-dimensional color space are identified as blue, red, and green.

The Weak Force And Decay

Quarks are the only elementary particles that experience all four fundamental interactions, including the weak interaction, which allows them to transform from one flavor to another. This flavor transformation is mediated by the emission or absorption of W bosons and is responsible for the radioactive process of beta decay, where a neutron splits into a proton, an electron, and an electron antineutrino. The relative tendencies of these transformations are described by the Cabibbo-Kobayashi-Maskawa matrix, a mathematical table that encodes the probability of one quark changing into another, with the actual probability depending on the masses of the quarks and the corresponding matrix entries. While the process of flavor transformation is the same for all quarks, each quark has a preference to transform into the quark of its own generation, and the links between the weak interaction matrices for quarks and leptons remain unclear. This mechanism is not only fundamental to the stability of matter but is also routinely used in medical applications such as positron emission tomography and in experiments involving neutrino detection. The existence of CP violation, a phenomenon where weak interactions behave differently when left and right are swapped, was explained by the theoretical model of the GIM mechanism and the subsequent discovery of the charm quark.

The Mass Of The Universe

The mass of a quark is a complex concept involving two distinct terms: current quark mass, which refers to the mass of a quark by itself, and constituent quark mass, which includes the mass of the gluon particle field surrounding the quark. Most of a hadron's mass comes from the gluons that bind the constituent quarks together rather than from the quarks themselves, as gluons possess energy known as quantum chromodynamics binding energy. For example, a proton has a mass of approximately 938 MeV, of which the rest mass of its three valence quarks only contributes about 9 MeV, with the remainder attributed to the field energy of the gluons. The Standard Model posits that elementary particles derive their masses from the Higgs mechanism, which is associated to the Higgs boson, and further research into the top quark's large mass might reveal more about the origin of mass for all elementary particles. This distinction between current and constituent mass highlights the profound role of the strong interaction in creating the mass of the visible universe, as the energy of the gluon field contributes far more to the total mass than the intrinsic mass of the quarks.

Exotic States Of Matter

Under sufficiently extreme conditions, quarks may become deconfined out of bound states and propagate as thermalized free excitations in a larger medium known as quark-gluon plasma. This theoretical phase of matter is believed to have existed in the period prior to 10^-6 seconds after the Big Bang, when the universe was filled with quark-gluon plasma because the temperature was too high for hadrons to be stable. Recent experiments at the Relativistic Heavy Ion Collider have yielded evidence for liquid-like quark matter exhibiting nearly perfect fluid motion, although a state of entirely free quarks and gluons has never been achieved. Given sufficiently high baryon densities and relatively low temperatures, possibly comparable to those found in neutron stars, quark matter is expected to degenerate into a Fermi liquid of weakly interacting quarks. This liquid would be characterized by a condensation of colored quark Cooper pairs, thereby breaking the local SU(3)c symmetry and creating a phase of quark matter that is color superconductive, allowing color charge to pass through it with no resistance. The exact conditions needed to give rise to these states remain unknown and are the subject of ongoing research, with estimates putting the needed temperature for quark-gluon plasma at approximately 2 trillion Kelvin.